The one thing people get backwards
Moisture content in wood is water weight divided by oven dry weight, not by wet weight. So a board at 75 percent is not three quarters water. Its water weighs 0.75 times what the dry wood weighs, which means water is 0.75 / 1.75, or about 43 percent of the total. Taking 75 percent of the green weight as water overstates it by three quarters.
The way through it is always the same. Divide the green weight by one plus the moisture content as a decimal, and that gives the oven dry weight, which never changes as the wood dries. Every other figure comes off that fixed number: water at any moisture content is the dry weight times that moisture content, and the weight at any moisture content is the dry weight times one plus it. A 1,875 pound load at 75 percent contains 1,071 pounds of dry wood, 804 pounds of water, and going to 8 percent it gives up 718 pounds, which is 86 gallons.
Why shrinkage does not start immediately
Water in wood sits in two places: free water in the cell cavities, and bound water in the cell walls. Free water leaves first and takes no dimension with it, because nothing structural is losing anything. Only once the cavities are empty and the walls start giving up bound water does the wood get smaller. The moisture content where that changeover happens is the fibre saturation point, and it is the reason a board can lose half its water and not move a thousandth.
Below that point the shrinkage is treated as proportional. If a species shrinks 8 percent tangentially from green to oven dry and the fibre saturation point is 30 percent, then at 8 percent moisture content it has done 8 x (30 - 8) / 30, which is 5.87 percent of its green width. That is the model this page uses, and it is a straight line through a relationship that is not perfectly straight. The numbers for your species come from a wood handbook, not from here.
Across, through and along
Wood does not shrink the same in all three directions, and the difference is large. Tangential movement, which is what a flatsawn board does across its width, is typically around twice radial. Along the grain it is nearly nothing in normal wood, which is why boards stay the length they were sawn. That two to one ratio between tangential and radial is also why flatsawn boards cup: the two faces of the board are at different distances from the pith and shrink by different amounts.
All of this is why sawing oversize is not optional. The page will work back from a finished size to the green size that lands on it, using your species figures. That is shrinkage allowance only, and it does not include the planing, the sawing variation or the flattening that a real board needs on top.
Where the rest of this lives
For seasonal movement in a finished piece between two in-service moisture contents, rather than green to dry, the page you want is the wood movement calculator, which carries species figures and works out slot sizes. The green weight of a stack sitting outside is the stacking and sticker calculator, and if that stack is going into a kiln, the turnaround is the kiln charge and throughput calculator. The green sawn size that all of this starts from is the log cut plan calculator. For firewood, where the same water arithmetic decides how much heat you get, the page is the firewood BTU and seasoning calculator.
Questions people ask
Can moisture content really be over 100 percent?
Yes, and in green sapwood it often is. Because the figure is water weight divided by oven dry wood weight, there is nothing stopping the water from outweighing the wood: at 150 percent the load is one and a half pounds of water for every pound of dry substance, which makes it 60 percent water by total weight. This trips people who assume a percentage has to be a share of a whole. It is a ratio to a fixed reference, and the reference is the dry wood.
My meter will not read the green boards. What do I put in?
Pin meters lose their usefulness well above the fibre saturation point, which is roughly where the interesting part of drying starts, so a green reading from one is not a measurement. The options are an oven dry test on an offcut, which is weighing a sample, drying it and weighing it again, or a green moisture content figure from a species reference. Both are better than a meter reading over 30 percent, and the second is what most people use.
Why does the shrinkage not start until the fibre saturation point?
Because the water above it is sitting loose in the cell cavities and it takes nothing with it when it leaves. The cell walls are still saturated and still their full size. Only once the cavities are empty does water start coming out of the walls themselves, and that is when the wood begins to get smaller. Practically, it means a stack can lose an enormous amount of weight in the first weeks with almost no dimensional change, and then move steadily after that.
How much should I saw oversize?
The page will tell you what the shrinkage costs you between the two moisture contents you enter, using the species figures you enter, and that is only part of the answer. On top of it you need whatever the planer takes, whatever variation your saw produces, and enough thickness to get out any cup or twist that develops while the board dries. How much that is depends on your mill, your species and how flat the boards stay. Shops arrive at their own numbers by drying a few packs and measuring what came out.
Does this tell me how long the drying will take?
No, and nothing about the load tells you that. Drying rate depends on species, thickness, air speed, temperature, humidity and the schedule being run, and the published drying times exist because the relationship is complicated enough that it has been measured rather than derived. This page answers a different question: how much water has to leave, which is fixed by the two moisture contents and does not change no matter how fast or slow you get there.